In an image forming apparatus including a development unit adopting a two-component development using toner and carrier, when duration of developer (two component developer) expires, errors in image density, such as fog, are generated. Therefore, when the duration of the developer expires, the developer needs to be discharged and replaced.
However, it is complicated and time-consuming to discharge and replace the developer. Therefore, these works are usually done by a maintenance specialist. This, however, causes a disadvantage to a user. Specifically, this causes a loss of time between the expiration of the duration and actual replacement of the developer.
In view of the above disadvantage, an image forming apparatus including automatic-developer-discharge means has been suggested. For example, Japanese Unexamined Patent Publication No. 89061/1994 (Tokukaihei 6-89061, published on Mar. 29, 1994) discloses a development unit including automatic-developer-discharge means. The automatic-developer-discharge means has two transport screws, and one of them, that is disposed closer to the bottom of the developer tank, has a diameter larger than the other, thereby improving efficiency of discharging developer.
Further, Japanese Unexamined Patent Publication No. 61958/2004 (published on Feb. 26, 2004) suggests an automatic developer discharge technique in which developer is automatically discharged from the development unit and new developer is automatically supplied.
Further, Japanese Unexamined Patent Publication No. 155460/2000 (published on Jun. 6, 2000) suggests another automatic developer discharge technique in which an output voltage of the toner density sensor is detected, thereby assuredly discharging the developer and constantly maintaining a predetermined amount of developer after new developer is supplied.
However, none of the above publications teaches controlling the process of developer discharging. Therefore, the techniques of the above publications have a problem that a carrier is shifted from a development sleeve to a photoconductor (so-called beads-carry-over).
In an image forming apparatus adopting the two-component development, developer is made of two kinds of components: carrier and toner, and therefore, depending upon a potential difference between the photoconductor and the development sleeve, it may cause shifting of toner onto the photoconductor (development, fog), shifting of carrier onto the photoconductor or even out of a developer tank (this is so-called beads-carry-over and will be referred to as beads-carry-over hereinafter). The carrier shifted onto the photoconductor is rubbed and collected by a cleaning blade.
If the amount of the beads-carry-over photoconductor increases, a surface of the photoconductor and an edge of the cleaning blade are damaged when the carriers are collected and removed by the cleaning blade. This causes some kind of error in image forming because of incomplete cleaning or the like. Recently, OPC (Organic Photoconductor) photoconductors have been popularly used as the photoconductor, which more easily causes the above problem.
FIG. 19 is a graph showing a relationship between (i) an amount of residual developer in a developer tank at the time when the developer (waste developer) is discharged from the developer tank, and (ii) an amount of the beads-carry-over (a number of carriers shifted (moved) to the photoconductor per area unit on a surface of the photoconductor). FIG. 20 is a graph showing a relationship, based on FIG. 19, between (i) a developer discharge time (elapsed time from when the developer discharge is started) and (ii) the amount of the beads-carry-over to the photoconductor.
As shown in FIG. 19, the amount of developer in the developer tank decreases with a decrease in the amount of developer shifted to the development sleeve (MG), and this also causes a decrease in congestion of magnetic chains, resulting in an increase of the amount of the beads-carry-over. When the developer in the developer tank decreases to a certain amount at which the magnetic chains on the development sleeve is brought out of contact with the photoconductor, the beads-carry-over stops.
Further, as shown in FIG. 20, as the developer discharging time increases, the amount of the beads-carry-over to the photoconductor becomes greater. When the developer just starts discharging, a sufficient amount of magnetic chains exist on the development sleeve (the congestion of the magnetic chains is high). Therefore, the amount of the beads-carry-over gradually increases. As more and more amount of developer is discharged, the congestion of the magnetic chains on the development sleeve becomes sparser and sparser. This results in a decrease in magnetic flux density. Consequently, magnetic suction force exerted on the carriers becomes weak. As a result, the amount of the beads-carry-over rapidly increases. When the amount of developer in the developer tank further decreases to a certain point at which the magnetic chain on the development sleeve completely disappears (when the developer in the developer tank decreases to a certain amount at which the magnetic chains on the development sleeve is brought out of contact with the photoconductor), the beads-carry-over stops.
As described above, as the amount of developer in the developer tank decreases, the congestion of the magnetic chains of developer on the development sleeve becomes sparse, and the amount of the beads-carry-over tends to increase.
Specifically, due to magnetic force of the development sleeve, carriers and toners form chains, and are kept on the development sleeve. When the chains are brought into contact with the photoconductor, a carrier is physically and electrostatically disconnected from the chains. At this time, if the congestion of the magnetic chains is high, the separated carrier can be magnetically collected by a neighboring magnetic chain. On the other hand, if the congestion of the magnetic chains is low, there would be a fewer number of neighboring magnetic chains for magnetically collecting the separated carrier, and therefore the carrier released from the magnetic retention moves to the photoconductor, resulting in the beads-carry-over.
As described above, the amount of developer in the developer tank decreases as the developer is discharged, and the amount of the beads-carry-over gradually increases. This causes scratches on a surface of the photoconductor, resulting in some kind of error in image forming because of incomplete cleaning or the like.
Meanwhile, in order to prevent the beads-carry-over, it may be suggested to separate the photoconductor and the developer tank during developer discharge. In this case, however, it is required to include a precise separation mechanism that can maintain a constant gap (air gap) between the photoconductor and the developer tank, making the mechanism complex and increasing production costs.
Moreover, if, during developer discharge, the developer tank is rotated while the photoconductor is stopped, the magnetic chain of the developer contacts with a limited part of the photoconductor. The part of the photoconductor that is rubbed by the developer is damaged. As a possible solution for this problem, the transport screw, which discharges the developer, and the development sleeve, which transports the developer to the photoconductor, may be separately driven in the developer tank. However, this mechanism is also complex and expensive.